Manipulation Of The Normal/reverse Selectivity Of Polyamide Thin-film Membranes For Biogas Upgrading

Biogas,a‘‘dormant’’byproduct derived from anaerobic digestion of organic waste,is one of the most promising candidates for the replacement of fossil fuels.Depending on the origin and season,biogas typically consists of methane（40–75Vol%）and some undesirable impurities such as carbon dioxide（25–60 Vol%）,water vapor,ammonia and hydrogen sulfide in trace amount.Among these impurities,CO₂as the most primary impurity provides no heating value but causes corrosion in industrial implementation.Therefore,CO₂removal has been considered as a crucial procedure in biogas purification in order to employ biogas reaching the same standards as natural gas.Considering biogas source is usually dispersed and the output of single gas source is limited,the separation process must be as simple and efficient as possible to save cost.Interfacial polymerization is a mature and feasible technique for large-area fabrication of ultrathin membranes.In this paper,we utilize Interfacial polymerization to prepare polyamide（PA）membranes and select suitable monomer,additive,and substrate pre-treatment measure to regulate the physicochemical property of membrane.We fabricate PA-based condensing-water membranes（CWMs）by condensing water on the PA composite membranes to improve the separation performance of instinct water membrane.Meanwhile,we found these membranes exhibit CH₄/CO₂reverse selectivity under certain film making and testing conditions,which has both scientific and technical significance.On one hand,CH₄/CO₂reverse selective membrane has an industrial value for recovering CH₄from lean gas.On the other hand,CH₄/CO₂reverse selectivity is more rare than N₂/CO₂reverse selectivity in a very few literatures.This is a challenge to solution-diffusion mechanism,which is the classical transport theory of small molecules through the membrane.What’t more,it is expected to promote the further development of mass transport theory in membrane.The main contents and conclusions are listed as follows:1.In this work,we design and fabricate PA-based condensing-water membranes（CWMs）by condensing water on the PA composite membranes to deal with aggressive biogas,relying on the differences of solubility and diffusivity in water between CH₄and CO₂.The optimizations of the pre-treatmeat approach of substrate,interfacial polymerization process and water management impart the resultant CWM a superior CO₂permeance of 374 GPU with CO₂/CH₄selectivity of 31.The gas separation performance of the resultant CWM exceed that of bulk water,which confirmes the synergistic effect between swollen PA layer and water layer.Furthermore,the gas separation performance of this CWM retains stable under high applied pressure（350 k Pa）in 16 days.In addition,the relevant parameters of PA CWM such as water layer thickness and the gas permeance of swollen PA supporting membrane under different humidification conditions are analyzed by means of an improved resistance model for multilayer composite membrane.2.By investigating the influence of membrane-fabricating parameters on the CH₄/CO₂selectivity,we found and formed the regulation method and theory of CH₄/CO₂reverse selectivity based-on PA membrane.Combined with sporadic cases reported in the literature,we propose that membranes with reverse selective usually have the common characteristics of high chain rigidity,rich polar groups,and wide transfer channels,in order to provide useful clues for solving the major scientific problem of reverse selectivity.